A significant advancement in fitting soft toric contact lenses is the ability to calculate cross-cylinder effects using a
sphero-cylinder over-refraction and a cross-cylinder calculator. Before the availability of these calculators, practitioners primarily determined lens power parameters using the "left add, right subtract" (LARS) method of adjusting for lens rotation. Such calculations are limited in that LARS will not reveal underlying:

errors in refraction

vertex calculation errors

possible cylinder masking in thicker/stiffer ballasted lenses

lens draping effects secondary to the underlying corneal topography

In other words, many unknown power variables may exist. A simple soft lens over-refraction can pick up virtually all of these variables and, when calculated properly, patients can achieve excellent visual results.

Figure 1. ToriTrack calculates the final prescription based on three
variables.

My Calculator of Choice

Several cross-cylinder calculators are available. The one I've found most useful is CooperVision's
ToriTrack. This calculator doesn't require on-eye estimation of lens rotation, which can be a significant source of error. The program can determine the resultant lens power to order based on three known variables: baseline manifest refraction, contact lens power and
sphero-cylinder over-refraction.

ToriTrack also helps in selecting appropriate empirical lens parameters based on corneal diameter, apical radius of curvature and refractive error. It selects a base curve based on the overall sagittal height of the cornea and suggests lens power to compensate for tear lens effects that occur with certain ballasted designs.

How it Works

You can find ToriTrack at
www.coopervision.com. If you're fitting a high modulus or thicker, ballasted lens design such as Proclear or Hydrasoft
(CooperVision), click the appropriate box and enter the spectacle prescription from the
phoropter. Click "calculate," and the prescription appears, including compensation for vertex distance and a potential tear lens. If you chose a low modulus or non-ballasted design, then the calculator suggests a power that doesn't require presumed lacrimal lens compensation.

After you apply the diagnostic lens and allow it to settle, perform a
sphero-cylinder over-refraction. Click "over-refraction" to enter this third variable. The program then calculates the final prescription (Figure 1). In this example, the patient requires a lower sphere and cylinder power in addition to a modified axis based on the cross cylinder effects from a lens that rotates.

If we applied LARS alone in this example, the patient would be over-corrected in both principal power meridians (although the axis might be close). The LARS method points us in the right direction only with respect to the axis; it doesn't help determine sphere or cylinder power.

Also Consider These

ToriTrack is limited in that it relies on a stable manifest refraction for baseline calculations. In cases of irregular astigmatism, consider using programs that ask for the amount and direction of lens rotation, such as those found on
www.eyedock.com or the Sunsoft Calculator (Ocular Sciences, Inc.).

Dr. Smythe is an associate professor of optometry at Pacific University and is in private group practice in Beaverton,
Oregon.